Mercury carbodiimide

Following our nomenclature, HgNCN(I) stands for the mercury carbodiimide (Scheme 3.1) and HgNCN(II) represents the mercury cyanamide (Scheme 3.2). In fact, the two different compounds (or two different solid-state isomers, as... 

Fig. 3. 42 Structural motifs from the crystal structures of mercury carbodiimide (a) and mercury cyanamide (b).

The first question of any solid-state chemist thinking in terms of structure and energetics will definitely be What is the stable polymorph, mercury carbodiimide, HgNCN(I), or mercury cyanamide, HgNCN(ll) Unfortimately, it is impossible to answer this question by means of differential thermal analysis because both polymorphs decompose, prior to interconversion, at about 230 °C to yield a white polymer and mercury metal. Thus, theoretical reasoning and/or electronic-structure theory is needed. Let us attempt to argue using both classical and quantum-chemical means. 

An alternative strategy for investigating the above trend focuses on molecules containing the structural units of mercury carbodiimide/cyanamide in the series of hypothetical oligomers -[Hg-N=C=N-] and -[Hg-N-C=N] (see Figure 3.44(a)) in which the chains are terminated with amine groups. 

Finally, let us not forget that there is a single, simple frame of understanding which correctly, albeit qualitatively, predicts the energetic order of mercury carbodiimide and mercury cyanamide, and it is based on chemical ideas, namely Pearson s concept of hard and soft acids and bases soft prefers soft and hard prefers hard. The absolute softness of Hg + (7.7 eV, see Table 2.3) lies between those of Pb and Ag+ such that a cyanamide anion and not a carbodiimide anion will be the preferred bonding partner for Hg +, just as for Pb + and Ag+. I cannot refrain from noting that I find this quite remarkable. 

The present procedure2 describes the conversion of resin-bound, primary aliphatic amines into isothiocyanates and the conversion of the latter into 3-aminothiophenes. The generation of isothiocyanates is related to known procedures,3 in which amines are first treated with carbon disulfide and the resulting dithiocarba-mates are desulfurized by treatment with a condensing agent (alkyl chloroformates, carbodiimides, lead or mercury salts, etc.). The presence of resin-bound isothiocyanates on the polystyrene support could be qualitatively ascertained by infrared spectroscopy (KBr-pellet strong absorption at 2091 cm-1). 

Behringer and Meier found that treatment of thiourea 105 with yellow mercury(Il) oxide gave traces of N, AT-pentamethylenecarbodiimide (107), the smallest known cyclic carbodiimide. Compound 107 was reported as unstable and able to polymerize readily (larger ring homologs were, ex-... 

Organomercury derivatives react with carbodiimides similarly. For example, phenyl-dichlorobromomethyl mercury 417 reacts with carbodiimides to give the expected insertion products 418. ... 

Transition metal carbodiimides, such as MnNCN and CuNCN, and carbodiimides derived from zinc, mercury,silver and thallium are also known. A preceramic polymeric titanium carbodiimide is obtained in the reaction of TiCU with bis(trimethyl-silyl)carbodiimide. Liganded carbodiimidotitanium complexes are obtained in the reaction of CpaTiCla with Me3SnN=C=NSi(i-Pr)3. ° Also, dicyclopentadienyl titanium (IV) diisocyanates are converted into carbodiimides with LiN(SrMe3)2. ... 

The reaction of Ar,A -bis(tert-butoxycarbonyl)thiourea (see Table 4) with amines in the presence of mercury(II) chloride or water-soluble carbodiimide provides an efficient method for preparation of the corresponding bis-protected guanidines.t An adaptation of this... 

Fig. 3.44 Structures of mercury cyanamide and carbodiimide oligomers (a) resembling the -Hg-NCN- chains in the extended structures of HgNCN(l)...

An interesting new method of synthesis of carbonimidoyl dichlorides involves the addition of dichlorocarbene to aliphatic carbodiimides. Thus, reaction of phenyl(bromodichloromethyl)mercury (LXV) (the dichlorocarbene precursor) with diisopropylcarbodiimide produces isopropyl-carbonimidoyl dichloride (LXVI) in 63% yield... 

The syntheses of substituted 5-aminotetrazoles 228, using azide ions for assembling the tetrazole scaffold, can be divided into two different approaches. The first approach is the addition of sodium azide to car-bodiimides 222 [156,157] or cyanamides 223 [158-161]. The second approach is the nucleophilic substitution of a chlorine in a-chloroformamidines 224 [162], the nucleophilic substitution of benzotriazole in (benzotria-zolyl)carboximidamides 225 [163], the nucleophilic substitution of the sxflfite anion in aminoiminomethanesulfonic acids 226 [164] and the nucleophilic substitution of sulfur from thioureas 227 in the presence of mercury [165] or lead [ 166-169] salts in the presence of an azide ion (Scheme 43). In this chapter, the main focus is on the addition of azides to carbodiimides 222 to form substituted 5-aminotetrazoles 228. 

Phenyl(trihalogenomethyl)mercury Iminocarbonylhalides from carbodiimides... 

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